1. Field of the Invention
The present invention relates to devices which are used for measuring the density of members, such as bones, and more particularly to devices which utilize ultrasonic acoustic signals to measure the physical properties and integrity of the members.
2. Description of the Prior Art
Various devices presently exist which may be used to measure the physical properties and integrity of a member such as a bone. Non-invasive density measuring devices can be used to determine cumulative internal damage caused by micro-crushing and micro-fracturing occurring in the bones of humans or animals such as race horses. Additionally, osteoporosis, or loss of bone mineralization, detection in humans and its cure or prevention are increasingly becoming areas of intense medical and biological interest. As the average age of the human population increases, a greater number of patients are developing complications due to rapid trabecular bone loss.
U.S. Pat. No. 3,847,141 to Hoop discloses a device for measuring the density of a bone structure, such as a finger bone or heel bone, to monitor the calcium content thereof. The device includes a pair of opposed spaced ultrasonic transducers which are held within a clamping device clamped on the bone being analyzed. A pulse generator is coupled to one of the transducers to generate an ultrasonic sound wave which is directed through the bone to the other transducer. An electric circuit couples the signals from the receive transducer back to the pulse generator for retriggering the pulse generator in response to those signals. The pulses therefore are produced at a frequency proportional to the transit time that the ultrasonic wave takes to travel through the bone structure, which is directly proportional to the speed of the sound through the bone. The speed of sound through a bone has been found to be proportional to the density of the bone. Thus the frequency at which the pulse generator is retriggered is proportional to the density of the bone.
Another device and method for, establishing, in vivo the strength of a bone is disclosed in U.S. Pat. Nos. 4,361,154 and 4,421,119 to Pratt, Jr. The device includes a launching transducer and a receiving transducer which are connected by a graduated vernier and which determine the speed of sound through the bone to determine its strength. The vernier is used to measure the total transit distance between the surfaces of the two transducers.
Lees (Lees, S. (1986) Sonic Properties of Mineralized Tissue, Tissue Characterization With Ultrasound, CRC publication 2, pp. 207-226) discusses various studies involving attenuation and speed of sound measurements in both cortical and spongy (cancellous or trabecular) bone. The results of these studies reveal a linear relationship between the wet sonic velocity and wet cortical density, and between the dry sonic velocity and the dry cortical density. The transit times of an acoustic signal through a bone member therefore are proportional to the bone density. Langton. et al. (Langton, C. M., Palmer, S. D., and Porter, S. W., (1984), The Measurement of Broad Band Ultrasonic Attenuation in Cancellous Bone, Eng. Med., 13, 89-91) published the results of a study of ultrasonic attenuation versus frequency in the os calcis (heel bone) that utilized through transmission techniques. These authors suggested that attenuation differences observed in different subjects were due to changes in the mineral content of the os calcis. They also suggested that low frequency ultrasonic attenuation may be a parameter useful in the diagnosis of osteoporosis or as a predictor of possible fracture risk.
The present invention provides an acoustic image of the human heel. The image can be used to provide greater insights into material and structural variations within the heel, to locate a consistent region of interest on a given heel, or to develop a template which can be used to improve the reproducibility of multiple measurements of a patient over several visits.
Specifically, the present invention provides an imaging ultrasonic bone densitometer with at least one ultrasonic transducer arranged to measure acoustic signals modified by different portions of the bony member. An electronic data processor receives the electrical signals corresponding to the acoustic signals and processes the signals to determine corresponding member variables related to the property of bony member at the different locations. A display communicates with the data processor to provide a measure of the bony member at the positions. The member variables may be attenuation, broad band ultrasonic attenuation (BUA), time of flight, speed of sound or a combination of these measurements.
It is thus one object of the invention to provide an ultrasonic bone densitometer providing a spatially sensitive information about bone quality.
The display may be a graphic display providing an image of the bony member, the image indicating the member variables as measured at the different locations.
It is thus another object of the invention to provide a densiometric image useful for evaluating bone quality.
The electronic data processor may operate to analyze the member variables to identify a measurement region of interest in the bone. The member variables within the region of interest may then be determined.
It is another object of the invention, therefore, to provide an imaging ultrasonic bone densitometer where the image data can be used to accurately locate a measurement region within the heel.
The densitometer may use an array of ultrasonic transducers providing a focused measurement of acoustic signals passing through a predetermined location within the bony member. The electronic data processor may scan the predetermined location through the bony member to provide a planar or volumetric image.
Thus it is another object of the invention to provide an ultrasonic bone densitometer that may produce a high resolution densiometric image. The predetermined location may be shifted electronically to obtain information for a complete image both across the transmission path of the ultrasonic signals and at different depths within the bone along the transmission path of the ultrasonic signal.
The electronic data processor may measure two locations within the bone, the first being within the trabecular region and the second at the cortical edge of the bony member.
Thus it is another object of the invention to provide a densitometer that may make two spatially separate measurements indicating different types of bone within its field of view.
The foregoing and other objects and advantages of the invention will appear from the following description. In this description, reference is made to the accompanying drawings which form a part hereof and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made therefore to the claims for interpreting the scope of the invention.